Microlithographic objective reducing lenses are used for reducing and imaging patterns for electronic microcircuits on semi-conductor wafers. The microlithography industry seeks ever-greater accuracy in the registration and imaging to facilitate miniaturizing the circuitry and to preserve accuracy and reliability in the final product. Hence, any substantial increase in accuracy for microlithographic objective reducing lenses is a welcome advance in this art.
Metrology for microlithographic objective reducing lenses is necessary for referencing the wafer relative to the lens to ensure that the desired interrelationship between the lens and the wafer is maintained as accurately as possible. Six degrees of freedom of relative motion are possible and include X and Y registration in the plane of the wafer surface, Z axis distance between the lens and the wafer, tip and tilt angles between the lens and wafer, and the rotational angle of the wafer around the Z axis of the lens. Computers are programmed to control the movements involved, and detectors or sensors for gathering information on the position of the wafer relative to the lens are included within the metrology components. These have previously been mounted on a frame or housing surrounding the barrel of the lens, and they have included such elements as microscope objectives for registering with location marks on the wafer surface, mirrors for reflecting the beams of fringe-counting interferometers, and a grazing incidence interferometer for detecting the Z axis distance between the wafer surface and the lens.
We have discovered a better and more accurate way of integrating the metrology components with a microlithographic objective reducing lens. Our metrology arrangement improves the attainable stability and accuracy, which we accomplish partly by integrating the metrology components with the barrel of the lens to eliminate instabilities caused by the previous metrology frames or housings around the lens barrels. We have also selected and arranged the metrology components that we integrate into the lens barrel to minimize distances between the lens and its metrology components and to ensure that everything cooperates for enhancing accuracy.